James K. Polk Sends General Zachary Taylor To Texas

The United States supported Texas when it claimed all land north of the Rio Grande, and this provoked a dispute with Mexico.

In June 1845, James K. Polk sent General Zachary Taylor to Texas, and by October, 3,500 Americans were on the Nueces River, prepared to defend Texas from a Mexican invasion. Polk wanted to protect the border and also coveted the continent clear to the Pacific Ocean. Polk had instructed the Pacific naval squadron to seize the California ports in case Mexico declared war. At the same time he wrote to Thomas Larkin, the American consul in Monterey, that a peaceful takeover of California would be welcomed.

In 1845, Texas became a U.S. state, and President James K. Polk directed Taylor to deploy into disputed territory on the Texas-Mexico border, under the order to defend the state against any attempts by Mexico to take it back after it had lost control by 1836. Taylor was given command of American troops on the Rio Grande River. When some of Taylor's men were attacked by Mexican forces near the river, Polk told Congress in May 1846 that a war between Mexico and the United States had started by an act of the former. That same month, Taylor commanded American forces at the Battle of Palo Alto, using superior artillery to defeat the significantly larger Mexican opposition. In September, Taylor was able to inflict heavy casualties upon the Mexican defenders at the Battle of Monterrey. The city of Monterrey was considered "un-destroyable". He was criticized for not ensuring the Mexican army that surrendered at Monterrey disbanded. Afterwards, half of Taylor's army was ordered to join General Winfield Scott's soldiers as they besieged Veracruz. Mexican General Antonio López de Santa Anna, through a letter by Scott destined for Taylor that had been intercepted by the Mexicans, found out that Taylor had only 6,000 men, many of whom were not regular army soldiers, and resolved to defeat him. Santa Anna attacked Taylor with 20,000 men at the Battle of Buena Vista in February 1847, inflicting 672 American casualties at a cost of 1,800 Mexican. As a result, Santa Anna left the field of battle.

After the Texas annexation, Polk turned his attention to California, hoping to acquire the territory from Mexico before any European nation did so. The main interest was San Francisco Bay as an access point for trade with Asia. In 1845, he sent diplomat John Slidell to Mexico to purchase California and New Mexico for $20–30 million. Slidell's arrival caused political turmoil in Mexico after word leaked out that he was there to purchase additional territory and not to offer compensation for the loss of Texas. The Mexicans refused to receive Slidell, citing a technical problem with his credentials. In January 1846, to increase pressure on Mexico to negotiate, Polk sent troops under General Zachary Taylor into the area between the Nueces River and the Rio Grande—territory that was claimed by both the U.S. and Mexico.